Another big tensor fix b9820

sched : reintroduce less synchronizations during split compute (#20793)

• CUDA: Improve performance via less synchronizations between token (#17795)
• Adds CPU-to-CUDA copy capability to ggml_backend_cuda_cpy_tensor_async()
• Adds function to relax sync requirements between input copies on supported backends (CUDA for now)
• Exchanges synchronous copy with async copy function.
• Adds macro guards to allow compilation in non-CUDA builds
• Reworked backend detection in ggml-backend.cpp to avoid linking conflicts
• Relax requirement of checks in async CUDA copies from backend and buffer type to just buffer type, to avoid linking issues
• Minor cleanup
• Makes opt-in to relax use of explicit syncs more general. Backends like vulkan which require a synchronization between HtoD copies and graph execution could also adopt this change now.
• Reintroduces stricter check for CPU->CUDA backend async copy via GGML_DEVICE_TYPE_CPU.
• Corrects initialization of ggml_backend_sync_mode in ggml_backend_sched_split initialization
• Simplifies synchronizations to adhere to saaasg pattern.
• Apply suggestion from u/ggerganov (src->buffer to buf_src)